I logged into the admin portion of my site today, and there was a warning regarding the version of php (5.4) on my site. I backed up my database and such, followed the instructions to run through setup again, and then received the fatal error:

"Fatal error: Call to undefined function zp_loggedin() in /home/mpifmetal/public_html/ferrous/zp-core/reconfigure.php on line 109"

I checked the setup.log and it says that:

Warn: Zenphoto core files [Some files are missing or seem wrong] Perhaps there was a problem with the upload. You should check the following files: themes/default/album.phpthemes/default/styles/light.cssthemes/default/styles/sterile-dark.cssthemes/default/styles/sterile-light.cssthemes/garland/album.phpthemes/garland/gallery.phpthemes/garland/image_gallery/functions.phpthemes/garland/zen.csszp-core/functions.phpzp-core/global-definitions.php

So I walked through: http://www.zenphoto.org/news/installation-and-upgrading#re-running-and-re-uploading-setup-files

and manually deleted and reinstalled index.php, /themes, and /zp-core.

Now when I go to my setup.php, I get a blank screen. The page reads that no data is being read, just an opened and immediately closed body tag.

Any idea what I should do next? Delete the entire zenphoto folders and start over? (I'd love to avoid this if possible)

Thank you for your input.

The warning you got first is more a note as explained on the page you linked.
Why you got a fatal error I cannot say but the 2nd warning is just that.

A blank screen can mean a lot of things. Probablywrong permissions on files/folders. I would indeed suggest to re-upload /zp-core at least to see if that fixes it. But in any case find the server error log to see if there is anything noted.